Carburetor



Feb. 15, 1966 R. E. KALERT, JR 3,235,236

CARBURETOR Filed Feb. 25. 1963 INVENTGR. RALPH El. KALERT JR.

United States Patent 3,235,236 CARBURETOR Ralph E. Kalert, Jr., GraniteCity, Ill., assrgnor to ACF Industries, Incorporated, New York, N.Y., acorporation of New Jersey Filed Feb. 25, 1963, Ser. No. 260,414 4Claims. (Cl. 261-41) This invention relates to carburetors, andparticularly to carburetors of the class having a diaphragm for controlof fuel admission thereto.

The invention is especially concerned with carburetors of the classdescribed for small internal combustion engines, such as are used onpower saws, for example, where ability to operate in any position, evenupside down, is important. For such applications, float-type carburetorsare generally unsuitable, and a diaphragm-type carburetor is preferred.A typical diaphragm-type carburetor has a fuel chamber closed by adiaphragm which is subject on the outside to atmospheric pressure. Thediaphragm actuates a valve controlling admission of fuel to the fuelchamber from a fuel supply. Fuel is supplied from the fuel chamber tothe mixture conduit of the carburetor through a so-called high-speedfuel system.

In the operation of such a carburetor, it is important that, when thethrottle of the carburetor is opened for acceleration of the engine andparticularly when the throttle is opened from idle position, fuel isimmediately delivered by the high speed fuel system into the mixtureconduit. Otherwise, there may be an undesirable lag in acceleration, andthe engine may even stall. Lag in delivery of fuel to the mixtureconduit may occur due to what may be referred to as excessive fuel lift,meaning excessively high pressure head to be overcome by the diaphragmbefore fuel is adequately delivered from the main nozzle of thehigh-speed fuel system of the carburetor. Lag in delivery of fuel mayalso occur due to What may be referred to as back bleeding, meaningdiminution of fuel in the high-speed fuel system due to loss of fuelfrom the high-speed system, during idle operation of the engine. Thus,when the throttle is opened for acceleration, there is a lag in deliveryof fuel through the main fuel system.

Accordingly, an object of this invention is the provision of acarburetor of the diaphragm type having a novel construction as tominimize fuel lift and back bleeding.

Another object of the invention is to provide novel carburetor structurefor rapid engine acceleration.

In general, the stated minimizing of fuel lift and back bleeding isaccomplished by a novel carburetor design in which the high-speed fuelsystem of the carburetor is taken directly from the fuel inlet of thecarburetor instead of from the diaphragm fuel chamber. This results in ashorter flow path of fuel to the nozzle and results in less lift indrawing fuel to the mixture conduit. Also, during idle or low speedoperation of the engine the novel design places all of the main fuelsystem at substantially atmospheric pressure so that there is notendency to back bleed.

The invention accordingly comprises the constructions hereinafterdescribed, the scope of the invention being indicated in the followingclaims.

In the accompanying drawings, in which several of various possibleembodiments of the invention are illustrated,

FIG. 1 is a longitudinal section of a conductor constructed inaccordance with this invention and connected to the intake manifold ofan engine.

FIG. 2 is a central transverse section of the carburetor of FIG. 1 takenon line 2-2, showing the high-speed fuel system of the carburetor.

ice

FIG. 3 is a transverse section taken on line 3-3 of FIG. 1, showing theidle fuel system of the carburetor.

FIG. 4 is a partial sectional view of the throttle and idle port of thecarburetor of FIGS. 1-3.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

Referring to the figures of the drawings, a carburetor constructed inaccordance with this invention and generally designated C comprises abody assembly 1 and a cylindrical metal tube assembly 3 forming themixture conduit of the carburetor. Tube 3 has an integral flange 5 atits outlet 6. Flange 5 is backed by a plate 7. Flange 5 and plate 7 areprovided with bolt holes for the reception of bolts for attaching thecarburetor to intake manifold 13 of an internal combustion engine E inthe usual manner. Tube 3 has at its other end a sleeve 14. with anintegral flange 15 backed by a plate 17 forming an inlet 18 to thecarburetor.

A throttle shaft 23 extends across the mixture conduit tube 3 adjacentits outlet 6 and carries a throttle valve 25 shown in a closed positionin FIGS. 1 and 4. Secured on one end of shaft 23 is a throttle operatinglever 27 engageable with a stop formed by an extension 31 on plate 7 todetermine the wide-open position of the throttle valve. A choke shaft 33extends across the mixture conduit tube 3 parallel to the throttle shaftand adjacent to the inlet 18 of the mixture conduit. Shaft 33 has achoke valve 35 fixed to it. Secured on one end of the choke shaft is achoke operating lever 37 having a portion 39 engageable with a stopformed by a tongue 41 on plate 17 to determine the wide-open position ofthe choke valve.

Body assembly 1 comprises a molded plastic body 53 formed with shallowcircular cup-shaped portion 61 forming a shallow circular fuel chamber55. A diaphragm 57 closes the fuel chamber. The peripheral wall of thecup-shaped portion 61 has an outwardly projecting fiared rim 63. Themargin of diaphragm 57 is clamped against the face of this rim by ashallow cup-shaped plastic cap 59 having an annular groove 65 in theinside of its pe ripheral wall receiving the rim 63. In assembling theparts, the diaphragm is positioned over the recess or cham ber 55 withits periphery on the face of rim 63, then cap 59 is snapped into placeover rim 63. The diaphragm consists of a circular piece of suitableflexible material such as, for example, a piece of thin fuel resistantsynthetic rubber. Cap 59 has a central aperture 711 exposing diaphragm57 to the atmosphere. Around aperture 71, the cap 59 has bosses 73 forlimiting the outward movement of the diaphragm 57.

The circular fuel chamber 55 has a recess 77 within body 53. Dependingfrom the bottom of recess 77 is a rib 79 of rounded contour moldedintegrally with the body 53. A stamped sheet metal lever 81 has a loopportion 83 receiving the rib 79. Lever 81 has one end portion 85extending into contact with the center of a circular sheet metal plate67 in contact with the adjacent surface of diaphragm 57. End portion 85of the lever is apertured to receive tongue 69 struck out of the plate67. At its other end, lever 31 is forked as indicated at 87 to fit intoan annular groove 39 in one end of a needle valve 91.

Body 53 includes a cylindrical boss 93 on one side of the body.Extending up into this boss from recess 77 of chamber 55 is a cavity 95of circular cross section accornmodating the needle valve 91. Boss 93has a reduceddiameter tubular end extension 97 with a passage 99 ofsmaller diameter than cavity 95 extending from cavity 95 throughextension 97. A ported resilient valve seat member 10]., made of fuelresistant synthetic rubber, is seated against a shoulder 103 at theupper end of cavity 95 around the lower end of passage 99. The needlevalve 91 has a pointed end engageable with this seat member. It also hasa collar 195 adjacent to this pointed end. A coil compression spring 107reacts from a spring retainer and valve guide member 1119, pressed inthe lower end of cavity 95, against collar 105 to bias the needle valve91 toward its closed position illustrated in FIG. 2. Valve guide 109 isa thick washer and guides the needle valve 91 in axial sliding movement.It also provides restricted communication between cavity 95 and fuelchamber 55.

A molded plastic fitting 111 has a snap fit on extension 97. Thisfitting has a lateral nipple 113 for connection of a fuel line L throughwhich fuel can flow by gravity from a fuel tank T. Fitting 111 holds inplace and closes one end of a cylindrical filter screen 115. The otherend of screen 115 extends into passage 99. Fuel flowing through screen115 passes into chamber 55 when the needle valve is open. As will bedescribed in greater detail, when the diaphragm 57 is moved into chamber55, lever 81 is rocked counterclockwise, as viewed in FIG. 2, about rib79, and acts to pull the needle valve 91 to open position for the flowof fuel from the tank T to the fuel chamber 55.

At the top of body 53, as viewed in FIGS. 13, there is an elongatedconcave arcuate surface 117 seating the mixture conduit tube 3. Tube 3extends across the top of the body 53 to one side of boss 93. Extendingupwardly from the bottom of surface 117 is a stem or post 119 moldedwith body 53 as an integral part thereof. This stem 119 is generally ofcircular cross section and somewhat tapered from its root in body 53toward its upper end. It extends through an aperture 121 in the bottomof tube 3 and its upper end engages the inside of the top of the tube 3.A screw 12 is threaded into the stem 119 through a hole 123 in the uppersurface of tube 3. An arcuate metal washer 127 and an annular sealingwasher 129 are interposed between the head of the screw 125 and the topof tube 3 to hold the tube 3 and body 53 in assembly. A gasket 131 isinterposed between the tube 3 and body 53 within the arcuate surface 117and has an opening registering with aperture 121 through which the postextends. Post 119 is located approximately midway between the throttleshaft 23 and the choke shaft 33, and is clear of the path of rotation ofthe throttle and choke valves 25 and 35. The post 119 restricts thepassage of air through tube 3 with a resulting venturi effect.

The high-speed fuel metering system of the carburetor is indicatedgenerally in FIGS. 1 and 2. Post 119 is formed with a circular passage135 extending through the post substantially parallel to the axis oftube 3 and adjacent to the bottom of tube 3. From this passage 135 thereis a second passage 137 along the axis of the post down to a crosspassage 139 in body 53 which extends transversely in respect to tube 3through body 53 to the cavity 95. The end of cross passage 139, whichopens into cavity 95, is tapered at 141 converging toward cavity 95. Ahigh-speed fuel metering screw 14-3 has its outer end threaded into theother end of cross passage 139 and has a reduced diameter elongatedshank 145 extending through the cross passage 139. The fuel threadedmetering screw 143 has a tapered end 147 which is thus adjustablerelative to the tapered end 141 of passage 139. A main fuel nozzle 149is pressed in passage 137 and projects upward into passage 135. Fuelnozzle 149 has a restricted outlet 151 opening into passage 135.

The idle fuel system of the carburetor is indicated generally at 153.Body 53 is formed with a passage 155 extending down from the arcuatesurface 117 and terminating short of chamber 55. This passage 155 isdownstream from post 119 and is closed at the top by gasket 131. Body 53has a groove 157 in the arcuate surface 117 extending laterally from theupper end of passage 155. Gasket 131 and tube 3 are formed withregistering holes forming an idle port 159 for flow of idle fuel fromgroove 157 into the tube. A passage 161 extends from the one side ofbody 53 (FIG. 3) to the passage 155, the inner end of passage 151 whichopens into passage 155 is tapered, as indicated at 163 and convergestoward passage 155. Extending up from fuel chamber 55 to passage 161adjacent the inner end of passage 161 is a channel 165. An idleadjusting screw 1137 has its outer end threaded into the outer end ofpassage 161 and has a reduceddiameter shank 169 extending through thepassage 161 with a tapered end 171 adjustable relative to the taperedend 153 of passage 151 for idle fuel material purposes.

Under engine idling conditions, throttle is in the FIG. 1 idle or closedposition, and choke in the FIG. 1 open position. Manifold vacuum inconduit 3 downstream from the closed throttle 25 induces flow of fuelfrom fuel chamber to the mixture conduit through fuel passages 165, 163,155, 157 and port 159. The tapered portion 163 of passage 161 withportion 169 of idle adjustiing screw 157 meters the flow of idle fuelinto conduit 3. Some air will bleed past the throttle for mixing withthis idle fuel, or an idle air bleed port may be provided for bleedingair from upstream of the throttle into the assage 157 in a known manner.

When the throttle 25 is opened for acceleration, air will flow throughconduit 3. Post 119 provides a restriction to this air flow and the airpassing through passage 135 of the post flows at an accelerated rate andprovides a sub-atmospheric pressure in the region of the main nozzle151. Fuel flows through passage 141 where the flow is metered by taperedportion 14-7 of the adjustment screw 143 and through passage 139 andnozzle 149 into the air passage 135.

As fuel flows from cavity 95, fuel pressure in the cavity drops.Diaphragm 57, subject on its outer side to atmospheric pressure, movesinwardly against the bias of spring 197, which is connected to diaphragm57 through lever 21 and needle valve 91. This inward movement of thediaphragm 57 moves needle valve 91 off of its seat 1111 and permits theflow of fuel into cavity to replace fuel flowing to chamber 55 or topassage 139. Normally, valve 91 is in an open position during operationof the engine for the flow of fuel from tank T at a rate necessary tomake up for the flow of fuel from cavity 95.

As explained above, the opening of the inlet valve 91 is due to thelowering of fuel pressure within the cavity 95 and the sensing chamber55. This drop in pressure results from the flow of fuel out through themain fuel system or the idle fuel system of the carburetor during engineoperation and the fuel pressure within chamber 55 falls to a level belowatmospheric at which the diaphragm under atmospheric pressure will moveinwardly against the bias of spring 197 to open the valve 105.

A chronic defect of carburetors of this type utilizing a fuel chamber,in which the pressure of the fuel is controlled by a diaphragm exposedto ambient air pressure, is the insensitiveness of the inlet valveopening mechanism. There is considerable loss in friction between thediaphragm plate 67 and lever 81 as well as between lever 81 and the end89 of the inlet valve 91 when valve 91 is operated. Furthermore, thevalve closing spring 107 must of necessity be of a certain strength toretain the needle valve 91 closed against the fuel pressure within theinlet line L. This fuel pressure in line L is due to gravity or thepressure of a fuel pump in the line L between the tank and carburetor.

A particular problem results during the idle or low speed operation of acarburetor. During idle, fuel is utilized by the engine at a low rateand this fuel is removed from the sensing chamber 55. However, the dropin fuel pressure in chamber 55 may not be effective in opening valve 91to replenish the idle fuel before air is first drawn into the main fuelnozzle 149 and through the fuel passages 139 into the cavity 95. Thepresence of this air prevents rapid acceleration of the engine when thethrottle is suddenly opened, as there is no fuel in nozzle 149 and fuelpassage 139 to be immediately available for acceleration upon theopening of the throttle. Thus, the engine may stumble and may stopbefore fuel can be drawn up through passage 139 and nozzle 149 into themixture conduit 3. This is an unwanted operation of the carburetor as insome installations, such as with chain saws, for example, it isnecessary and very desirable that instant acceleration of the enginetake place upon the manual opening of throttle 25. Oftentimes, thesudden opening of the throttle is accompanied by application of a loadto the engine and, with a complete lack of fuel in the main fuelpassages of nozzles 149 and 139, acceleration is not obtained and theengine frequently stalls out. In some types of carburetors where themain fuel and the idle fuel passages are interconnected, air drawn intothe main fuel passages during idle, as described above,

will also pass into the idle system and out the idle port.

Such air bleeding into the idle circuit causes an unduly lean idlemixture and the engine will stop because of lack of fuel.

In accordance with this invention, the main fuel passages are connectedadjacent to the inlet valve seat 101. As shown in FIG. 2, the reducedpassage 141 leading into the main fuel passage 139 of the carburetor ispositioned as close to the inlet needle seat 101 as possible.Furthermore, the guide 109 is provided with a small clearance so thatthe area between the inner periphery of guide 109 and the surface of theneedle valve 91 is minimized, and is only slightly larger in area thanthat of the smallest passage 163 in the idle circuit. For example, in acarburector of the type described, the minimum area of the taperedpassage 163 is around 0.00085 square inch and the area of the spacebetween the guide 109 and the peripheral surface of the valve 91 may beas large or slightly larger. For example, the outer surface of theneedle valve 91 may have an outside diameter of 0.170 inch, while theinner diameter of the guide 109 would be in the order of 0.173 inch.This difference leaves a space between the guide 109 and the surface ofvalve 91, which is as large as that of the restriction 163 in the idlepassage.

Fuel from tank T or from a fuel pump in line L may have several poundsof pressure above atmospheric at the valve seat opening 101. Duringnormal operation of the carburetor, either at low speed or at high speedoperation, fuel is caused to flow out of the main nozzle 149 or the idleport 159 by the low pressure in the mixture conduit 3. This results in alowered pressure in the sensing chamber 55, which provides an opening ofthe needle valve 91 by atmospheric pressure against diaphragm 57. Thusduring engine operation there is a pressure drop from several poundsabove atmospheric upstream of valve seat 101 to a subatmosphericpressure in the sensing chamber 55 around /2 inch of water. The greatestdrop in fuel pressure is across the valve seat 101. During idleoperation with closed throttle, fuel flows through cavity 95 into thesensing chamber 55 and out the low speed fuel passages 165, 161, 155 and157. At this time, because of the fuel restriction between guide 109 andthe surface of valve 91, there is a drop in fuel pressure between thatin cavity 95 and the fuel pressure in the sensing chamber 55. With fuelpressure upstream of valve seat 101 above atmospheric and fuel pressurein chamber 55 at subatmospheric, it is believed that during this lowspeed operation, fuel pressure within cavity 95 is very close toatmospheric and that fuel at atmospheric pressure exists in a regionimmediately downstream of valve seat 101 within the cavity 95.

Thus, in accordance with the invention, the inlet 141 of the main fuelpassage is placed within the cavity region 95 and close to thedownstream side of the valve seat 101. This places entrance passage 141Within this region of fuel under atmospheric pressure. This arrangementeliminates air bleeding through nozzle 149, during idle or low speedoperation of the engine, when the throttle 25 is closed, since fuel inthe main fuel. nozzle will be under substantially atmospheric pressureequal for practical purposes to the pressure of the fuel at the entranceto the main fuel passage in the restricted region 141. Thus, with thenozzle 149 and region 141 at substantially the same pressure, there isno tendency for a flow of fuel between these points during low speed oridle operation of the engine.

Thus, the subatmospheric pressure in the sensing chamber is noteffective in pulling fuel out of the main fuel passages of nozzle 149and passage 139. Rather, instead, fuel is retained in these portions ofthe main fuel passage and, upon sudden opening of the throttle, fuel isreadily available in both the nozzle 149 and the fuel passage 139. Thisfuel with flow quickly into the mixture conduit 3 and sustain a rapidacceleration of the engine and prevent it from stalling out under asudden increase in load conditions.

In carburetors of the type described above, utilizing a diaphragmsensing chamber, it has been customary to connect the main fuel passagedirectly into the sensing chamber 55 so that fuel flowing to the mainnozzle during wide open throttle operation passes not only through acavity similar to 95 of applicants structure but also through thesensing chamber corresponding to 55 of the described invention. Thisresults in an extensive path of fuel flow from the inlet valve seat tothe main fuel nozzle.

A long fuel passage will provide a greater resistance to fuel flow thana correspondingly shorter passage. Greater resistance to fuel flow inthe main fuel passage requires a greater suction or lift at nozzleentrance 151 to draw fuel into the air passage 135. Thus, small changesin the fuel requirements of the engine are not sensed as quickly as whena fuel path of less resistance to fuel flow is used.

Therefore, in accordance with this invention, an advantage is obtainedby placing the entrance to the main fuel passage 14-1 as close to theinlet valve seat 101 as feasibly possible so that the fuel path from theinlet valve seat 101 to the main fuel nozzle 149 is minimized. As shownin FIG. 2, the flow of fuel from the valve seat 101 through passage 139into the main fuel nozzle 149 is of considerably less distance than ifthe fuel passed through the cavity 95 and through the sensing chamber 55before it flows into the restricted passage 141 as in some prior artcarburetors. This results in a lowered resistance to fuel flow andprovides greater sensitivity of the diaphragm to the requirements of theengine. Fuel will pass out of the nozzle 149 at lower vacuum ordepression pressures within the air passage than would be possible if alonger flow path oifering greater resistance to fuel flow. Also, changesin pressure at the nozzle entrance are reflected more quickly back tothe sensing chamber 55 and thus enable the diaphragm and lever as semblyto respond to smaller pressure differentials be tween the fuel insensing chamber 55 and the atmosphere. The carburetor described willthus respond quicker and to a greater extent to rapid accelerationrequirements of the engine.

In view of the above, it willbe seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

I claim:

1. A carburetor comprising a body formed with an air and fuel mixtureconduit therethrough, a throttle valve within said mixture conduitmounted for movement between an open and closed position, a choke valvewithin said mixture conduit spaced from said throttle valve and mountedfor movement between an open and a closed position, a fuel inlet, saidbody having a fuel sensing chamber, a flexible diaphragm forming a wallportion of said chamber, an elongated cavity forming an extension ofsaid chamber and having means defining a restricted fuel inlet passageat one end connection with said fuel inlet, a valve member having aconical shape with the apex of the valve member extending into therestricted passage and movable axially of said cavity to open and closesaid restricted passage, means biasing said valve member toward closedposition, said valve member being spaced from the side walls of saidcavity to define the between a fuel passageway for the flow of fuel fromsaid fuel inlet to said fuel sensing chamber, means forming an operativeconnection between said diaphragm and said valve member to move saidvalve member in an open direction against said biasing means, meansforming a restricted outlet fuel passage at the other end of said cavitybetween said cavity and sand sensing chamber, air flow restricting meanswithin said mixture conduit including a tubular post having an airpassage therethrough and aligned with the direction of air flow throughsaid mixture conduit, said post extending transversely across saidmixture conduit, a main fuel nozzle extending into said air passage andhaving an outlet in said air passage, a main fuel passageway connectingsaid fuel passageway between the restricted fuel inlet opening and therestricted fuel outlet opening with said main fuel nozzle, said mainfucl passageway including an inlet opening of tapered form and ahigh-speed fuel metering screw having a conical extremity which extendsinto the tapered inlet opening for regulating the flow of fueltherethrough, a port opening into said mixture conduit adjacent to saidthrottle in closed position, and an independent idle fuel passageextending only from said fuel sensing chamber directly to said port.

2. The invention of claim 1 wherein the elongated cavity is of circularcross-section and the valve member positioned therein is likewise ofcircular cross-section but of a smaller diameter and an annular memberis fixed to the lower end of said cavity with its inner wall spaced fromthe valve member to provide a restricted passageway for the fluid fromthe cavity to the sensing chamber.

3. The invention of claim 2 wherein said annular men.- ber has its upperwall forming a seat for a coil spring providing the means for biasingthe valve towards closed position.

4. The invention of claim 1 wherein the idle fuel passage is providedwith an adjustable metering means for regulating the flow of idle fuelthrough the idle fuel passage.

References Cited by the Examiner UNITED STATES PATENTS 2,680,605 6/1954Bracke. 2,823,905 2/1958 Brown. 3,093,699 6/1963 Demitz 261-'723,118,009 1/1964 Phillips.

HARRY B. THORNTON, Primary Examiner.

RONALD R. WEAVER, Examiner.

1. A CARBURETOR COMPRISING A BODY FORMED WITH AN AIR AND FUEL MIXTURECONDUIT THERETHROUGH, A THROTTLE VALVE WITHIN SAID MIXTURE CONDUITMOUNTED FOR MOVEMENT BETWEEN AN OPEN AND CLOSED POSITION, A CHOKE VALVEWITHIN SAID MIXTURE CONDUIT SPACED FROM SAID THROTTLE VALVE AND MOUNTEDFOR MOVEMENT BETWEEN AN OPEN AND A CLOSED POSITION, A FUEL INLET, SAIDBODY HAVING A FUEL SENSING CHAMBER, A FLEXIBLE DIAPHRAGM FORMING A WALLPORTION OF SAID CHAMBER, AN ELONGATED CAVITY FORMING AN EXTENSION OFSAID CHAMBER AND HAVING MEANS DEFINING A RESTRICTED FUEL INLET PASSAGEAT ONE END CONNECTION WITH SAID FUEL INLET, A VALVE MEMBER HAVING ACONICAL SHAPED WITH THE APEX OF THE VALVE MEMBER EXTENDING INTO THERESTRICTED PASSAGE AND MOVABLY AXIALLY OF SAID CAVITY TO OPEN AND CLOSESAID RESTRICTED PASSAGE, MEANS BIASING SAID VALVE MEMBER TOWARD CLOSEDPOSITION, SAID VALVE MEMBER BEING SPACED FROM THE SIDE WALLS OF SAIDCAVITY TO DEFINE THEREBETWEEN A FUEL PASSAGEWAY FOR THE FLOW OF FUELFROM SAID FUEL INLET TO SAID FUEL SENSING CHAMBER, MEANS FORMING ANOPERATIVE CONNECTION BETWEEN SAID DIAPHRAGM AND SAID VALVE MEMBER TOMOVE SAID VALVE MEMBER IN AN OPEN DIRECTION AGAINST SAID BIASING MEANS,MEANS FORMING A RESTRICTED OUTLET FUEL PASSAGE AT THE OTHER END OF SAIDCAVITY BETWEEN SAID CAVITY AND SAND SENSING CHAMBER, AIR FLOWRESTRICTING MEANS WITHIN SAID MIXTURE CONDUIT INCLUDING A TUBULAR POSTHAVING AN AIR PASSAGE THERETHROUGH AND ALINGED WITH THE DIRECTION OF AIRFLOW THROUGH SAID MIXTURE CONDUIT, SAID POST EXTENDING TRANSVERSELYACROSS SAID MIXTURE CONDUIT, A MAIN FUEL NOZZLE EXTENDING INTO SAID AIRPASSAGE AND HAVING AN OUTLET IN SAID AIR PASSAGE, A MAIN FUEL PASSAGEWAYCONNECTING SAID FUEL PASSAGEWAY BETWEEN THE RESTRICTED FUEL INLETOPENING AND THE RESTRICTED FUEL OUTLET OPENING WITH SAID MAIN FUELNOZZLE, SAID MAIN FUEL PASSAGEWAY INCLUDING AN INLET OPENING OF TAPEREDFORM AND A HIGH-SPEED FUEL METERING SCREW HAVING A CONCIAL EXTREMITYWHICH EXTENDS INTO THE TAPERED INLET OPENING FOR REGULATING THE FLOW OFFUEL THERETHROUGH, A PORT OPENING INTO SAID MIXTURE CONDUIT ADJACENT TOSAID THROTTLE IN CLOSED POSITION, AND AN INDEPENDENT IDLE FUEL PASSAGEEXTENDING ONLY FROM SAID FUEL SENSING CHAMBER DIRECTLY TO SAID PORT.